In ATM networking, cells are transmitted over virtual connections. Virtual connections represent stateful communication setups such as an ATM virtual circuit or an Internet TCP connection. At each end of the network virtual connection is a software application that can send and receive messages. The messages are carried across the network as packets or frames that are further subdivided into 48 byte ATM cells. The interface in and out of the forwarding device is either 48 byte ATM cells or 64 byte frame segments. Each virtual connection has a quality of service or rate specification. ATM Forum Traffic Management Specification 4.1 specifies the types of rates, e.g. constant bit rate (CBR), variable bit rate (VBR), unspecified bit rate (UBR), etc. Unspecified bit rate can have a priority associated with the virtual connection.
Network devices such as routers, switches, and traffic shapers schedule the transmission of cells to a network. One form of schedule for transmission is the calendar schedule, where a slot of the schedule represents a period of time for possible transmission of one or more cells. A virtual connection is “scheduled” according to a calendar schedule if a slot in the calendar schedule is reserved for the virtual connection. A transmission process performs the transmissions of the network device. The transmission process uses the calendar schedule as a guide for when to offer transmission opportunities to scheduled virtual connections.
Groups of virtual connections may be scheduled for transmission to one region of a network, going through a network interface such as a port. A large traffic shaper may handle many schedules. For example, each schedule may be for a different port or network domain.
Like reference symbols in the various drawings indicate like elements.